A magnetoresistive (MR) sensor exhibits a change in electrical resistance as a function of an external magnetic field. This property allows MR sensors to be used as magnetic field sensors and read heads in magnetic storage systems including disc drives and random-access-memories.
In disc drive storage systems, the read head is typically merged with a writer head. The writer writes encoded information to a magnetic storage medium, which is usually a disc coated with hard magnetic films. In a read mode, a magnetic domain representing a bit of data on the disc modulates the resistance of the MR sensor as the magnetic domain passes below the read head. The change in resistance can be detected by passing a sensing current through the MR sensor and measuring the voltage across the MR sensor. The resultant signal can be used to recover the recorded data from the disc.
MR sensors utilize various MR effects, such as giant magnetoresistance (GMR) and tunneling magnetoresistance (TMR). The structure of the MR sensor varies depending upon the MR effect being utilized. GMR sensors in the form of “spin valves” are generally favored by the disc drive industry. Spin valves generally consist of a free ferromagnetic layer having a magnetization that rotates in response to an applied magnetic field, a conductive spacer, and a pinned ferromagnetic layer whose magnetization has a fixed orientation. The electrical resistance of the spin valve is a function of the angle between the magnetizations of the free ferromagnetic layer and the pinned ferromagnetic layer. The spin valve is most resistive when the two layers are magnetized in anti-parallel directions, and is the most conductive when they are parallel.
A TMR sensor utilizes a TMR junction that is very similar to a spin valve in the sense that it also consists of a ferromagnetic free layer, a spacer, and a pinned ferromagnetic layer. The magnetoresistance effect rises from the angular difference between the magnetizations of the two magnetic layers in a way that is analogous to the spin valve. A major difference between the TMR junction and the spin valve is that the spacer in the TMR junction is made of an insulator, typically aluminum-oxide, instead of a conductor. Moreover, in conventional TMR sensors, the electrical current is perpendicular to the plane of the films as opposed to in the plane of the films for GMR sensors.
There is a never-ending demand for higher data storage capacity in disc drives. One measure of the data storage capacity of a disc drive is the areal density of the bits at which the disc drive is capable of reading and writing. The areal density is generally defined as the number of bits per unit length along a track (linear density in units of bits per inch) multiplied by the number of tracks available per unit length in the radial direction of the disc (track density in units of track per inch or TPI).
A goal of present magnetic recording research is to achieve terabit (1012)-per-square-inch areal density. Such a high areal density requires a significant decrease in the size of the magnetic domains that define the bits of data, which also reduces the magnitude of the magnetic field they generate. Accordingly, the read sensor that is used to detect the magnetic field must be highly sensitive (i.e., exhibit a large magnetically induced change in resistance in response to an applied magnetic field) in order to properly detect the magnetic domains. Unfortunately, the sensitivities of GMR sensors (approximately 25% maximum resistance change) and TMR sensors (approximately 40% maximum resistance change) are believed to be insufficient for use in reading data that has been recorded at a terabit areal density.
One promising MR effect that could be used to form a read sensor having a sufficient sensitivity to enable reading of terabit areal density magnetic recordings is the ballistic magnetoresistance (BMR) effect. Such BMR sensors have exhibited sensitivities that are on the order of a 3,000% magnetically induced change in resistance in response to an applied magnetic field. The BMR effect occurs in the conduction of spin-polarized electrons between magnetic leads through a highly constricted magnetic junction having a width of approximately 10 nanometers (nm). The width of the constricted junction restricts the magnetic domain wall of the constricted junction to less than the spin-flip mean free path of the electrons. When a magnetic domain wall resides in the constricted junction, the electrical resistance is much larger than it is after an external magnetic field is applied to substantially sweep out the domain wall. The resulting magnetoresistive effect is much larger than the GMR or TMR effects.
The primary obstacle that must be overcome to form such a sensor is the formation of the constricted junction. One method involves stretching a magnetic metal rod until the desired constricted junction forms without breaking the rod. Another method involves electro-deposition of magnetic material between adjacent tips of magnetic leads until the tips are joined by the deposited material. Unfortunately, such methods are difficult to perform, produce inconsistent results, can degrade rapidly (electro-deposition method), and are generally unacceptable for mass production.
Accordingly, a need exists for MR sensors having constricted junctions that can be formed small enough to produce a BMR effect while allowing for their mass production.